Ionization chambers ("ion chambers") were developed for measuring the strength of ionizing radiations such as x-rays and the radiation emitted by radioactive sources. Ion chambers are particularly useful for calibrating radiation sources to be used in radiotherapy--the treatment of cancer with radiation. One type of radiotherapy is known as "brachytherapy," in which one or more radioactive sources are inserted into the human body, either through body orifices or surgical procedures, to position them in or near a cancer tumor which may be thereby reduced or eliminated. Conventional brachytherapy involves some hazard to medical personnel due to radiation exposure incidental to the installation of the sources in the patient, and patient care during the several days' treatment duration.
The new brachytherapy technique of high-dose-rate remote afterloading eliminates that exposure by allowing personnel to install only non-radioactive plastic catheters. After medical personnel have moved out of the treatment room, a small but intensely radioactive gamma-ray source fastened to the end of a stiff wire is moved under computer control out of its lead shield and through the catheter (or sequentially through several catheters) to the desired treatment locations. The computer selects the exact source locations and dwell times within each catheter, according to the treatment program specified by the radiotherapist. The treatment duration is measured in minutes instead of days, because of the high source strength. The fact that only a single source is used, and that the delivery rate of dose (i.e., energy spent per unit mass of tissue) is high, makes imperative the accurate and reliable calibration of the source strength by means of a suitable chamber.
High dose rate remote afterloading brachytherapy devices, such as the Microselectron manufactured by the Nucletron Corporation, are being increasingly utilized in the United States. The most common radionuclide used in these devices is iridium 192 (.sup.192 Ir) in the form of a small pellet (e.g., 0.5 mm diameter 4 mm active length with 0.3 mm stainless steel wall) connected to a wire that pushes and pulls the pellet through a 2 mm outside diameter plastic catheter to guide the pellet to the desired locations. The initial activity of these sources is in the neighborhood of 10 curies (Ci), or 3.7.times.10.sup.11 becquerels (Bq). The half-life of .sup.192 Ir is approximately 74 days, requiring relatively frequent (usually quarterly) source replacement to maintain short treatment times. These sources must be calibrated when they are placed into use. The supplier of the sources provides a calibration certificate which states an overall uncertainty in activity of plus or minus 10%; thus, an independent recalibration is preferably carried out after installation in an afterloading unit at a hospital or outpatient cancer treatment facility.
A rigorous procedure for carrying out such a calibration involves first sending a small (less than a few cubic centimeters) thimble-shaped ion chamber to the National Institute of Standards and Technology ("NIST," formerly known as the National Bureau of Standards) for calibration. Since NIST does not offer ion chamber calibrations for gamma rays having the spectrum of iridium-192, an interpolation between two adjacent calibration energies that are offered is utilized. This chamber is then positioned at one or more known distances 10-40 cm from the iridium source in the afterloading catheter. The observed ion currents are small (of the order of 10.sup.-11 amperes) hence difficult to measure accurately due to electrical insulator leakage and other perturbations. Other significant sources of error include scattered radiation from the room, inaccurate distance measurement, effects of size and shape of the ion chamber, irradiation of the chamber during transit time of the source to and from its assigned location, and air temperature and pressure. Such a calibration requires extreme care to obtain accuracy closer than 1-2%. It is a task for a well-trained expert radiation physicist, not a radiotherapy technician. Thus it should not be attempted by the smaller radiotherapy facilities not served by a physicist. Moreover it is an inefficient use of professional personnel to require that such a complicated, time-consuming and error-prone procedure be repeated on a routine basis four times a year, even where competent personnel are available.
Consequently there is a need for a compact, rugged and accurate device for conveniently and reproducibly calibrating the strength of a radioactive high-dose-rate source such as Ir-192 while positioned in its guiding catheter, after the source has been installed in a brachytherapy afterloading unit at a radiotherapy facility.